i would say if you go nvidia 560 or better and ATI 6870 or better. 2 gig gddr5 is almost a requirement now for some games, both of those cards have 2gig options, thought the 6870 comes with 1 gig by default.
though if you can stretch your budget your the 7850 is a great steal around the 220 dollar mark on new egg, its about as powerful as a 560 ti 448 or a 6970
if you do go the build your own route buy a modular psu so much easier to work with, seasonic or corsair are both great brands
and i would recommend a sandybridge/ivybridge i5 for gaming over the 6core amd, your frame rate will suffer a bit using the amd chip (not major but will be negative)
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